1. Field of the Invention
This invention relates generally to radiation therapy equipment for the treatment of tumors, or the like, and specifically to a computerized method of evaluating dose levels in electron beam radiotherapy treatment.
2. Background Art
Medical equipment for radiation therapy treats tumorous tissue with high energy radiation. The amount of radiation and its placement must be accurately controlled to ensure both that the tumor receives sufficient radiation to be destroyed, and that damage to the surrounding and adjacent non-tumorous tissue is minimized.
Internal source radiation therapy places capsules of radioactive material inside the patient in proximity to the tumorous tissue. Dose and placement are accurately controlled by the physical positioning of the isotope. However, internal source radiation therapy has the disadvantage of any surgically invasive procedure, including discomfort to the patient and risk of infection.
External source radiation therapy uses a radiation source that is external. The source of the high energy radiation may be x-rays, or electrons from linear accelerators in the range of 2 to 25 MeV, or gamma rays from highly focused Co.sup.60 sources having an energy of 1.25 MeV. The external source is collimated to direct a beam into the patient to the tumor site.
Although the size and strength of the radiation beam from the external source may be accurately controlled outside of the patient, the dose received by a given volume within the patient may vary because of scattering and absorption of the radiation by the intervening tissue. For this reason, a determination of the proper dose and placement of the dose requires an estimation of the effects of treated tissue and the tissue surrounding the treated area in scattering or attenuating the radiation beam.
With electron beam radiotherapy equipment, the scattering of the electrons as they interact with medium is described by Fermi's partial differential equation: EQU .PSI..sub.z =-.alpha..PSI..sub.x +(T/4).PSI..sub..alpha..alpha. -.beta..PSI..sub.y +(T/4).PSI..sub..beta..beta. ( 1)
where .PSI. is a function of the Cartesian coordinates x, y and z describing the density of electrons (flux) at points within the volume of the medium and of angles .alpha. and .beta. which describe the trajectory of the electrons with respect to the z axis, and where .PSI..sub.z, .PSI..sub.x, and .PSI..sub.y are the partial derivatives of .PSI. with respect to x, y and z; .PSI..sub..alpha..alpha. and .PSI..sub..alpha..alpha. are the second partial derivative of .PSI. with respect to angles .alpha. and .beta. respectively, and T is the scattering power of the medium through which the radiation travels. In general, T will vary throughout the volume and will therefore be a function of x, y and z.
Analytic solutions to Fermi's partial differential equation, that is, algebraic functions .PSI.(x,y,z,.alpha.,.beta.) that meet the requirements of equation (1), are difficult to determine in all but a few simple cases. Those simple cases include that where the patient is homogeneous or where the values of T varies only as a function of z. Neither of these cases accurately describe many volumes within a patient where radiotherapy may be required.
Failure to accurately account for variations in the value of T in the x and y directions may lead to "hot" or "cold" spots where substantially more or less radiation is delivered than desired. This is especially true in the neighborhood of substantial x and y variations in density such as may occur if the tumor to be treated is near a cavity or bone.
For this reason, a "Monte Carlo" method is presently preferred for the calculation of radiation dose for electron beam radiotherapy. In this technique, the paths of several million particles are traced through a model of the patient which accurately reflects the three dimensional variations in the value of T within the volume being studied. At regular points along the path of each electron as modeled, a probability of scattering is calculated based on the value of T at that point. For large numbers of electrons (above 10.sup.6) an accurate representation of the dose is obtained.
Unfortunately, the Monte Carlo method is extremely time consuming and may take upwards of several hours for the computation of a single dose on current electronic computers.
The need exists for a method of dose modeling accurately accounting for variations in T and that has greater computational efficiency than the Monte Carlo technique.